This invention relates to magnetorheological dampers and, in particular, to the surface finish of piston rods operating in said dampers.
Magnetorheological (MR) dampening devices are used in various applications, such as dampers, shock absorbers, and elastomeric mounts, for dampening and controlling vibration. Devices utilizing the unique properties of magnetorheological fluids are used to control pressure in valves and to control the transfer of torque in brakes and clutches.
Magnetorheological (M) fluids are fluid compositions that undergo a change in apparent viscosity in the presence of a magnetic field. A typical MR fluid contains ferromagnetic microparticles suspended in a low viscosity carrier liquid which are capable of becoming polarized in the presence of an applied magnetic field. The particles become organized into chains of particles within the fluid. The particle chains increase the apparent viscosity (flow resistance) of the fluid. The particles return to an unorganized state when the magnetic field is removed, which lowers the viscosity of the fluid.
FIG. 1 shows a known monotube MR damper 10 for use in the suspension system of a vehicle having a piston 12 sliding within a hollow tube 14 filled with MR fluid 16. The piston 12 is attached to a hollow rod 18, referred to herein as the piston rod, that slides within a sealed bearing 20 at one end of the body of the damper 10. The piston 12 contains a coil 22, which carries a variable current, thus generating a variable magnetic field across a flow gap 24 between an inner core 26 and an outer shell or flux ring 28 of the piston 12. A bearing 30 having relatively low friction is disposed between the flux ring 28 and the tube 14. The flux ring 28 and the inner core 26 of the piston 12 are held in place by spoked end plates 32. Terminals 34 of the coil 22 extend through the piston rod 18 and are provided with suitable insulation for connection to a source of electricity (not shown). One end 36 of the tube 14 is filled with inert gas which is separated from the MR fluid 16 by a floating piston or sealed gas cap 38. The floating gas cap 38 accommodates the displacement of MR fluid 16 due to the varying length of piston rod 18 immersed within the MR fluid 16 of hollow tube 14 as the piston 12 moves and to accommodate thermal expansion of the MR fluid 16. The circumference of the gas cap 38 includes an o-ring 40 that provides a fluid-tight sealing engagement with the hollow tube 14. The hollow tube is sealed by end caps 42,44 and attachment eyes 46,48 are provided on the respective end caps 42,44 for installing MR damper 10 to a vehicle body (not shown).
In response to vibration-induced movement of the piston rod 18, MR fluid 16 flows through the flow gap 24. When the coil 22 is energized, the effective viscosity of the MR fluid 16 in the flow gap 24 is increased by the interaction of the microparticles with the applied magnetic field. Variations in the electrical current flowing to coil 22 can be used to modulate the strength of the applied magnetic field and, thereby, to control the apparent viscosity of the flowing MR fluid 16. The modulation of the apparent viscosity affects the flow rate of the MR fluid 16 through the flow gap 24 to achieve a desired dampening effect.
The MR fluid 16 provided in the hollow tube 14 comprises a plurality of soft ferromagnetic microparticles that are dispersed and suspended in a base liquid, preferably in a low viscosity base liquid. Suitable microparticles include powders of carbonyl iron, magnetite, iron alloys (such as those including aluminum, silicon, cobalt, nickel, vanadium, molybdenum, chromium, tungsten, manganese and/or copper), iron oxides, iron nitrides, iron carbides, chromium dioxide, low carbon steel, silicon steel, nickel, cobalt, and other materials known to exhibit MR activity.
A suitable microparticle size exhibits multi-domain characteristics when subjected to a magnetic field. For spherical or near-spherical particles, a suitable size distribution for the microparticles ranges between nominal diameters of about 1 and about 25 xcexcm, usually between about 1 xcexcm and about 6 xcexcm. The microparticles are preferably present in an amount between about 50 and 90 percent by weight of the total composition of the MR fluid 16. Suitable base liquids include hydrocarbon oil, silicone oil, paraffin oil, mineral oil, chlorinated and fluorinated fluids, kerosene, glycol, or water. A particularly suitable MR fluid 16 comprises carbonyl iron powder suspended in a synthetic hydrocarbon oil.
Because the microparticles are quite small, they have a tendency to become trapped in valleys that are created in the surface of the piston rod during the superfinishing or microfinishing process. The trapped particles are then dragged past the damper seal 20. The microparticles in the MR fluid 16 are highly abrasive and can damage the seal 20. As a result, the MR fluid 16 can eventually escape through the degraded seal and, ultimately, the MR dampening device can prematurely fail.
There is thus a need to prevent the damper seal from being damaged by particle abrasion from the MR fluid and moving piston rod.
The present invention provides a piston rod for use in a magnetorheological dampening device that has a surface finish that renders the dampening device resistant to wear at the elastomeric seal/piston rod interface. To this end, and in accordance with the present invention, the piston rod has a surface finish of Ra less than 0.065 xcexcm and xcex94axe2x89xa61.4xc2x0, as measured using a Gaussian filter with a 0.08 mm cut-off length. A magnetorheological dampening device operating with a piston rod having the surface finish of the present invention resists wear from an MR fluid having particles of less than about 16 xcexcm in diameter in fluid communication with the piston rod and elastomeric seal. The present invention further provides a method of achieving the surface finish, including rotating the piston rod while moving an abrasive tape against the outer surface of the rod.